Ol'skool 2004 P4 still feels pretty fast and smooth. Plays anything from it's era- Far Cry, FEAR, HL2, LOtR:TBfME, etc at 1280x1024 max settings. 4Ghz capable but the rise in voltage and heat required is not worth it, so 3.6ghz @ stock vcore is what it runs 24/7.
Before you ask- no this is not my main rig. Just an old system that is still enjoyable to own and use.